Re: [PATCH] spapr: Migrate CAS reboot flag

[Greg Kurz]

On Wed, 22 Jan 2020 17:50:28 +1100
David Gibson <address@hidden> wrote:

> On Tue, Jan 21, 2020 at 10:32:55AM +0100, Greg Kurz wrote:
> > On Tue, 21 Jan 2020 14:43:32 +1100
> > David Gibson <address@hidden> wrote:
> > 
> > > On Mon, Jan 20, 2020 at 09:04:38AM +0100, Greg Kurz wrote:
> > > > On Fri, 17 Jan 2020 16:44:27 +0100
> > > > Greg Kurz <address@hidden> wrote:
> > > > 
> > > > > On Fri, 17 Jan 2020 19:16:08 +1000
> > > > > David Gibson <address@hidden> wrote:
> > > > > 
> > > > > > On Thu, Jan 16, 2020 at 07:29:02PM +0100, Greg Kurz wrote:
> > > > > > > On Thu, 16 Jan 2020 13:14:35 +0100
> > > > > > > Greg Kurz <address@hidden> wrote:
> > > > > > > 
> > > > > > > > On Thu, 16 Jan 2020 11:37:24 +0100
> > > > > > > > Laurent Vivier <address@hidden> wrote:
> > > > > > > > 
> > > > > > > > > On 16/01/2020 09:48, Greg Kurz wrote:
> > > > > > > > > > On Wed, 15 Jan 2020 19:10:37 +0100
> > > > > > > > > > Laurent Vivier <address@hidden> wrote:
> > > > > > > > > > 
> > > > > > > > > >> Hi,
> > > > > > > > > >>
> > > > > > > > > >> On 15/01/2020 18:48, Greg Kurz wrote:
> > > > > > > > > >>> Migration can potentially race with CAS reboot. If the 
> > > > > > > > > >>> migration thread
> > > > > > > > > >>> completes migration after CAS has set spapr->cas_reboot 
> > > > > > > > > >>> but before the
> > > > > > > > > >>> mainloop could pick up the reset request and reset the 
> > > > > > > > > >>> machine, the
> > > > > > > > > >>> guest is migrated unrebooted and the destination doesn't 
> > > > > > > > > >>> reboot it
> > > > > > > > > >>> either because it isn't aware a CAS reboot was needed 
> > > > > > > > > >>> (eg, because a
> > > > > > > > > >>> device was added before CAS). This likely result in a 
> > > > > > > > > >>> broken or hung
> > > > > > > > > >>> guest.
> > > > > > > > > >>>
> > > > > > > > > >>> Even if it is small, the window between CAS and CAS 
> > > > > > > > > >>> reboot is enough to
> > > > > > > > > >>> re-qualify spapr->cas_reboot as state that we should 
> > > > > > > > > >>> migrate. Add a new
> > > > > > > > > >>> subsection for that and always send it when a CAS reboot 
> > > > > > > > > >>> is pending.
> > > > > > > > > >>> This may cause migration to older QEMUs to fail but it is 
> > > > > > > > > >>> still better
> > > > > > > > > >>> than end up with a broken guest.
> > > > > > > > > >>>
> > > > > > > > > >>> The destination cannot honour the CAS reboot request from 
> > > > > > > > > >>> a post load
> > > > > > > > > >>> handler because this must be done after the guest is 
> > > > > > > > > >>> fully restored.
> > > > > > > > > >>> It is thus done from a VM change state handler.
> > > > > > > > > >>>
> > > > > > > > > >>> Reported-by: Lukáš Doktor <address@hidden>
> > > > > > > > > >>> Signed-off-by: Greg Kurz <address@hidden>
> > > > > > > > > >>> ---
> > > > > > > > > >>>
> > > > > > > > > >>
> > > > > > > > > >> I'm wondering if the problem can be related with the fact 
> > > > > > > > > >> that
> > > > > > > > > >> main_loop_should_exit() could release qemu_global_mutex in
> > > > > > > > > >> pause_all_vcpus() in the reset case?
> > > > > > > > > >>
> > > > > > > > > >> 1602 static bool main_loop_should_exit(void)
> > > > > > > > > >> 1603 {
> > > > > > > > > >> ...
> > > > > > > > > >> 1633     request = qemu_reset_requested();
> > > > > > > > > >> 1634     if (request) {
> > > > > > > > > >> 1635         pause_all_vcpus();
> > > > > > > > > >> 1636         qemu_system_reset(request);
> > > > > > > > > >> 1637         resume_all_vcpus();
> > > > > > > > > >> 1638         if (!runstate_check(RUN_STATE_RUNNING) &&
> > > > > > > > > >> 1639                 !runstate_check(RUN_STATE_INMIGRATE)) 
> > > > > > > > > >> {
> > > > > > > > > >> 1640             runstate_set(RUN_STATE_PRELAUNCH);
> > > > > > > > > >> 1641         }
> > > > > > > > > >> 1642     }
> > > > > > > > > >> ...
> > > > > > > > > >>
> > > > > > > > > >> I already sent a patch for this kind of problem (in 
> > > > > > > > > >> current Juan pull
> > > > > > > > > >> request):
> > > > > > > > > >>
> > > > > > > > > >> "runstate: ignore finishmigrate -> prelaunch transition"
> > > > > > > > > >>
> > > > > > > > > > 
> > > > > > > > > > IIUC your patch avoids an invalid 'prelaunch' -> 
> > > > > > > > > > 'postmigrate' runstate
> > > > > > > > > > transition that can happen if the migration thread sets the 
> > > > > > > > > > runstate to
> > > > > > > > > > 'finishmigrate' when pause_all_vcpus() releases the main 
> > > > > > > > > > loop mutex.
> > > > > > > > > > 
> > > > > > > > > > ie. symptom of the problem is QEMU aborting, correct ? The 
> > > > > > > > > > issue I'm
> > > > > > > > > > trying to fix is a guest breakage caused by a discrepancy 
> > > > > > > > > > between
> > > > > > > > > > QEMU and the guest after migration has succeeded.
> > > > > > > > > > 
> > > > > > > > > >> but I don't know if it could fix this one.
> > > > > > > > > >>
> > > > > > > > > > 
> > > > > > > > > > I don't think so and your patch kinda illustrates it. If 
> > > > > > > > > > the runstate
> > > > > > > > > > is 'finishmigrate' when returning from pause_all_vcpus(), 
> > > > > > > > > > this means
> > > > > > > > > > that state was sent to the destination before we could 
> > > > > > > > > > actually reset
> > > > > > > > > > the machine.
> > > > > > > > > 
> > > > > > > > > Yes, you're right.
> > > > > > > > > 
> > > > > > > > > But the question behind my comment was: is it expected to 
> > > > > > > > > have a pending
> > > > > > > > > reset while we are migrating?
> > > > > > > > > 
> > > > > > > > 
> > > > > > > > Nothing prevents qemu_system_reset_request() to be called when 
> > > > > > > > migration
> > > > > > > > is active. 
> > > > > > > > 
> > > > > > > > > Perhaps H_CAS can return H_BUSY and wait the end of the 
> > > > > > > > > migration and
> > > > > > > > > then be fully executed on destination?
> > > > > > > > > 
> > > > > > > > 
> > > > > > > > And so we would need to teach SLOF to try H_CAS again until it 
> > > > > > > > stops
> > > > > > > > returning H_BUSY ? It seems safer to migrate the CAS reboot 
> > > > > > > > flag IMHO.
> > > > > > > > 
> > > > > > > 
> > > > > > > Ok I've tried that with a patched SLOF that sleeps 500ms and 
> > > > > > > tries CAS
> > > > > > > again if H_BUSY was returned. It fixes the issue but it looks a 
> > > > > > > bit
> > > > > > > ugly because of the polling with an arbitrary timeout in SLOF... 
> > > > > > > I'm
> > > > > > > not very comfortable either with calling migration_is_active() 
> > > > > > > from
> > > > > > > the CAS code in QEMU.
> > > > > > > 
> > > > > > > David,
> > > > > > > 
> > > > > > > Any suggestion ?
> > > > > > 
> > > > > > Yeah, I think looping in SLOF is a worse idea than migrating the
> > > > > > cas_reboot flag.
> > > > > > 
> > > > > > But.. a better solution still might be to just remove the remaining
> > > > > > causes for CAS reboot entirely.  CAS reboots pretty much suck when
> > > > > > they happen, anyway.
> > > > > > 
> > > > > 
> > > > > I Agree.
> > > > > 
> > > > > > With the irq changeover condition removed, I think the remaining
> > > > > > causes are more theoretical than practical situations at this point.
> > > > > > 
> > > > > 
> > > > > FWIW, hotpluggging a PCI device before CAS result in a hung guest 
> > > > > (not yet
> > > > > investigated the details).
> > > > 
> > > > commit 10f12e6450407b18b4d5a6b50d3852dcfd7fff75
> > > > Author: Daniel Henrique Barboza <address@hidden>
> > > > Date:   Wed Aug 30 15:21:41 2017 -0300
> > > > 
> > > >     hw/ppc: CAS reset on early device hotplug
> > > > 
> > > > I'll have a look to see what can be done here.
> > > 
> > > Ah.. yes, that one might be a bit tricky.
> > > 
> > 
> > So far it seems to be related to SLOF not being able to create
> > new nodes in the DT when parsing the FDT returned by CAS. SLOF
> > stops the parsing and returns an error. The guest ends up with
> > a broken DT and eventually hangs later (in my case the kernel
> > believes it is going to do hash while radix was negotiated with
> > QEMU). I need to dig some more.
> 
> Uh... I don't think this is right.  I'm pretty sure SLOF *does* create
> new nodes when parsing the CAS FDT, it will need to for
> "ibm,dynamic-reconfiguration-memory" at least.
> 

It can create "memory@" or "ibm,dynamic-reconfiguration-memory" nodes but
explicitly rejects all others.

> I've done some looking and I think the actual reasons here are a bit
> more complicated (but also possibly easier to handle).
> 
>   1. We can't send hotplug events to the guest until after CAS,
>      because before then we don't know if it can process them, or if
>      it can, which interrupt it uses for them.
> 
>   2. Queueing hotplug events across CAS for delivery afterwards
>      introduces other complications
> 
>   3. We need to make sure that each device appears exactly once in
>      either the  initial device tree that the guest OS sees, *or* in a
>      hotplug event, not both or neither.
> 
> Now that we rebuild the DT at CAS time, I think we mightd be able toy
> handle this by converting such devices to "cold plugged" at CAS time
> (similarly to what we already do at reset).  Since they're in the
> CAS-time DT which is what the OS will consume, cold plugged is
> effectively how the OS will see them.
> 

I have tried hacking around to achieve that. Basically calling
spapr_drc_reset() for all DRCs for which spapr_drc_needed()
returns true.

> A remaining problem is that new PCI devices won't get BARs assigned by
> SLOF in this scenario.  We'll probably get away with it because of the
> linux,pci-probe-only property, but I don't know we want to rely on

We currently only expose this property for pseries-4.2 and newer
machine types... this could be a problem.

> that.  PCI bridges hotplugged introduce further complications, because
> they won't get enumerated.
> 
> > > > But I agree the other check is more theoretical:
> > > > 
> > > >     /* capabilities that have been added since CAS-generated guest 
> > > > reset.
> > > >      * if capabilities have since been removed, generate another reset
> > > >      */
> > > >     spapr->cas_reboot = !spapr_ovec_subset(ov5_cas_old, spapr->ov5_cas);
> > > > 
> > > > Unless changing kernels or tempering with the kernel command line, I 
> > > > don't
> > > > see how some capabilities could change between the two CAS in practice.
> > > 
> > > Well, we want to be robust and it's at least theoretically possible
> > > that the guest will request different things on subsequent reboots.
> > 
> > Yes but in the latter case a full machine reset occurs and
> 
> Not necessarily.  A guest could ask for something on one CAS cycle,
> then reject it on another, without doing a full reboot.  It'd be a
> pointless thing for the guest to do, but it's possible.
> 

Ok, I was asking later on if we want to support the scenario of
multiple CAS without an intermediate full reboot. I now have the
answer :)

> > spapr->ov5_cas gets cleared, ie. spapr_ovec_subset() returns
> > true in the check above no matter what.
> 
> Well, also it could happen if the guest rejects something we put in
> the initial value of ov5_cas (which is populated from spapr->ov5, so
> it's not entirely empty).
> 

AFAICT the initial value of ov5_cas after a full machine reset is
all zeroes until CAS does:

    /* full range of negotiated ov5 capabilities */
    spapr_ovec_intersect(spapr->ov5_cas, spapr->ov5, ov5_guest);

which is done between:

    ov5_cas_old = spapr_ovec_clone(spapr->ov5_cas);

and

    spapr->cas_reboot = !spapr_ovec_subset(ov5_cas_old, spapr->ov5_cas);

So I don't quite understand how ov5_cas_old, ie. spapr->ov5_cas at the
time the guest calls CAS, could have an "initial value not entirely
empty"... This can only happen if the guest calls CAS several times
without doing a full reboot in between. My initial thought was to
refuse this scenario and fail any subsequent CAS attempt made by
the guest before a full reboot.

> > > However I believe that the original rationale for this check was that
> > > while we could add things to the device tree for added capabilities,
> > > we didn't have a way to roll back the changes for removed
> > > capabilities.
> > > 
> > 
> > IIUC this is specifically for "removed capabilities since last
> > CAS". This can happen if:
> > 1) we're already processing a CAS reboot or,
> > 2) a freshly rebooted guest calls CAS twice without being rebooted
> >    in between.
> > 
> > Since a freshly booted or rebooted guest can only trigger a CAS
> > reboot because of a "hotplug-before-CAS", if we manage to get rid
> > of this limitation, 1) cannot happen anymore.
> > 
> > The linux kernel seems to be only calling "ibm,client-architecture-support"
> > once during early boot so 2) should _never_ happen. Do we care to support
> > this scenario anyway ?
> 
> I think you've missed some things in your reasoning.  But it doesn't
> really matter because the full dt rebuilt should handle it anyway.  I
> have a draft patch which removes this cause for CAS reboots.
> 
> Still grappling with the hotplug-before-CAS case.
> 

Same here actually. I was struggling with SLOF to have it create new nodes
for hotplugged-before-CAS devices without crashing :-\

I think I'll wait for your patches to arrive :) Please Cc: me.

> > > Now that we fully rebuild the device tree at CAS, I think this test
> > > can probably just go, although there's some double checking to do.
> > > 
> > 
> > I tend to agree.
>

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